1. Field of the Invention
The present invention relates to a brightness enhancement film, an optical sheet member, and a liquid crystal display device. More specifically, the present invention relates to a brightness enhancement film which has high brightness and is able to suppress an oblique change in the shade at the time of being incorporated in a liquid crystal display device, an optical sheet member using the brightness enhancement film, and a liquid crystal display device using the brightness enhancement film.
2. Description of the Related Art
A flat panel display such as a liquid crystal display device (hereinafter, also referred to as LCD) has been annually variously used as a space saving image display device having low power consumption. The liquid crystal display device, for example, is configured by disposing backlight (hereinafter, also referred to as BL), a backlight side polarizing plate, a liquid crystal cell, a visible side polarizing plate, and the like in this order.
In the recent flat panel display market, power saving, high definition, and enhancement in color reproducibility have progressed as enhancement in LCD performance in both large-size application mainly for TV and middle-size and small-size application for a tablet PC or a smart phone.
It has been proposed that a reflection polarizer is disposed between the backlight and the backlight side polarizing plate according to power saving of the backlight. The reflection polarizer is an optical element in which among incident light rays vibrating in all directions, only light rays vibrating in a specific polarization direction are transmitted, and light rays vibrating in the other polarization direction are reflected. Increasing brightness (the degree of brightness per unit area of a light source) by solving low light efficiency of LCD has been expected as a main component for a low power LCD according to an increase in mobile devices and low power consumption of home electric appliances.
In response, in JP3448626B, a technology is disclosed in which an optical sheet member (a dual brightness enhancement film (DBEF: Registered Trademark) or the like) is combined between the backlight and the backlight side polarizing plate, and thus, a light utilization rate of the BL is improved by recycling light, and the brightness is improved while saving power of the backlight.
In JP1989-133003A (JP-H01-133003A), a technology is disclosed in which a reflection range broadens by a reflection polarizing plate configured by laminating a λ/4 plate and a layer formed by immobilizing a cholesteric liquid crystalline phase and three or more layers formed by immobilizing cholesteric liquid crystalline phases in which the pitches of the cholesteric liquid crystalline phases are different from each other, and thus, a light utilization rate of the BL is enhanced by recycling light.
Here, when the reflection polarizing plate configured by laminating the λ/4 plate and the layer formed by immobilizing the cholesteric liquid crystalline phase is incorporated in the liquid crystal display device, it has been known that a change in the shade (also referred to as shade unevenness) when viewed from an oblique direction which is caused by optical properties of the cholesteric liquid crystalline phase and the λ/4 plate easily occurs. In response, in JP3518660B, a method in which the pitch of the cholesteric liquid crystalline phase shortens at an incidence side of the light, and disposing a compensation layer having a refractive index in a vertical direction which is greater than an in-plane refractive index are proposed. In addition, in WO2008/016056A, a method is proposed in which retardation of the λ/4 plate in a thickness direction is set to be less than 0.
In addition, a method is proposed in which a plurality of layers having different pitches are disposed as a polarizing plate using a layer formed by immobilizing other cholesteric liquid crystalline phases in order to broadening the reflection range, and the pitches are gradually changed. In JP4570377B, simply and reliably forming a liquid crystal layer having a structure in which a spiral pitch of the liquid crystal layer is gradually changed in the thickness direction by applying a liquid crystal composition containing a compound having a fluorinated alkyl group onto a transparent substrate, and integrating a retardation element having specific Rth with a ¼ wavelength plate are proposed.
Furthermore, when the layer is formed by immobilizing the cholesteric liquid crystalline phase, in general, a rod-like liquid crystal compound is used as a cholesteric liquid crystal material, and the same reflection function as that of a case of using the rod-like liquid crystal compound is able to be obtained by aligning a disk-like liquid crystal into the shape of a spiral. The disk-like liquid crystal having a spiral structure, for example, is disclosed in JP2001-81465A.
On the other hand, a method has been also known in which a light emitting spectrum of the backlight becomes sharp from the viewpoint of high definition and enhancement in color reproducibility in the liquid crystal display device. For example, in JP2012-169271A, a method is disclosed in which white light is embodied by using a quantum dot (hereinafter, also referred to as QD) emitting red light and green light between a blue LED and a light guide plate as a fluorescent body, and thus, high brightness and enhancement in color reproducibility are realized. In SID'12 DIGEST p. 895, a method of combining a light conversion sheet (QDEF, also referred to as a quantum dot sheet) using a quantum dot for enhancing color reproducibility of the LCD is proposed.